California Transportation Department officials at a meeting of the Metropolitan Transportation Commission on Wednesday said that faulty bolts being used in the construction of a new span of the Bay Bridge might have been damaged by rainwater, reports KQED’s Rachel Dornhelm.

They also said they can’t promise the bridge will be ready for its scheduled Labor Day opening.

The bolts are intended to stabilize the bridge in the event of an earthquake. Earlier reports focused on the manufacturing process. In fact, some questions came up about the way the bolts were made by Dyson Corp. of Ohio, and hydrogen in the bolts appears to have made them brittle.

But engineers in the meeting raised a new possibility: The hydrogen could have come from rainwater gathering in the holes where the bolts sat after being installed.

When workers tried to tighten 96 bolts between March 8 and March 15, 32 of them snapped. One bit of evidence supporting the rainwater theory is that the flaws were in the bottoms of the bolts where water is most likely to have collected.

But the investigation is not yet complete.

Also at the MTC meeting, engineers said they had not yet figured out how to compensate for the broken bolts. They mentioned an external harness and additional concrete as two of the possibilities under consideration.

Since they don’t know how they’re going to fix the problem, the Caltrans engineers said they can’t guarantee the bridge will open as scheduled on Labor Day.

That concerned Oakland Mayor Jean Quan and other leaders at the meeting because festivities are planned for that date.

The Caltrans engineers said it might be possible to start using the bridge even before the bolt problem is fixed because the new span would still be stronger than the bridge that drivers are using now.

They did have some good news: Another batch of 192 bolts from the same manufacturer, Dyson Corp., have stood up to stress tests. These bolts were made in 2010. The ones that broke were made in 2008.